It is known to make medical devices, such as catheters, visible during magnetic resonance imaging by applying a paramagnetic or ferromagnetic marker band to the device. The ferromagnetic or paramagnetic materials used in the marker band disturb the magnetic field in a nuclear magnetic resonance field so as to produce a visible image on the viewing screen of a magnetic resonance imaging device.
Typically, medical catheters follow a tortuous, or winding, path as the device is inserted into the blood vessel of a subject. The distal tip of the catheter is generally flexible to avoid damage to the inner walls of the blood vessel as the catheter is passed through the vessel. The distal tip of the catheter is also generally pre-bent to a desired configuration so that the catheter may be inserted into branching blood vessels along the path. When the tip is pre-bent, the physician must be able to orient the tip so that it can be pushed into the branching blood vessels.
Representative prior art patents that disclose flexible, elongated medical catheters are U.S. Pat. No. 5,045,072 to Castillo and U.S. Pat. No. 5,171,232 to Castillo. Each of these patents, assigned to the assignee of the present invention, discloses a medical catheter in which the very distal tip of the catheter is impregnated with a radiopaque material so that when the catheter is viewed by x-ray radiation, or alternatively by fluoroscopy, the radiopaque tip is made visible so that an attending physician may view the distal tip of the catheter.
When medical catheters are used in conjunction with a magnetic resonance imaging machine, it is necessary to apply a marker band or region of a paramagnetic or ferromagnetic material to cause that band or region of the catheter to become visible under a nuclear magnetic resonance field.
One problem with currently available catheters and other medical devices which are used in a nuclear magnetic field is that as the medical device is moved or rotated to different positions within the patient so that the medical device is viewed from different angles, the level of disturbance of the magnetic field created by the magnetic material changes rather dramatically. As the surface area of the marker which is "viewed" by the magnetic resonance imaging equipment changes, i.e., increases or decreases, there is a corresponding increase or decrease in the brightness of the visual image on the viewing screen. In other words, if the medical device is rotated such that the imaging marker presents a very large surface area when "viewed" by the equipment, a very bright image appears on the imaging screen. Similarly, if the medical device is rotated to a position in which the surface area as "viewed" by the imaging device becomes very small, the image presented on the viewing screen may be either very weak or non-existent.
As may be appreciated, with the existence of a very large and intense visual image on the viewing screen it is possible that certain surrounding objects may be entirely obscured. Alternatively, if the surface area as "viewed" of the imaging marker is very small, it is possible that the visual image may be difficult, if not impossible to detect by the physician.